This invention relates to a position measuring device, and particularly to a photoelectric angle measuring device for measuring the angular position of two relatively rotatable objects.
Position measuring devices are used, for example, in processing machines for measuring the relative position of a tool and workpiece. Typically, a scanning unit, connected to the tool, scans the graduation plate connected to the workpiece. The signal obtained as a result of scanning is used to determine the angular position of the tool with respect to the workpiece.
The dissertation, "Photoelectric Measuring of the Change of Lengths or Angular Positions with the Help of Diffraction Grids," by F. Hock, 1975, describes and shows, in FIG. 86, an angle measuring device for the elimination of the angular pitch of a graduation plate. In this device, a light beam from a light source traverses, by means of a condenser, a first angular graduation area of an angular graduation of the graduation plate. The beam is then conducted through a first pentagonal prism, a first deviation prism, and a first objective lens to a Wollaston prism. The prism splits the beam into two partial beams. Both partial beams pass through a second objective lens, a second pentagonal prism, a second deviation prism, and a second angular graduation area located diametrically opposite the first angular grauuation area. Two photoreceivers receive the partial beams by means of a polarizing separator prism. A disadvantage is that, due to the number of optical elements, this device is dimensionally large, and is expensive to assemble and adjust.
In the article "Laser Rotary Encoder," by Nishimura et al., "Motion," July/August, 1986, an angle measuring device for the elimination of eccentricity errors in the angular graduation pitch of aggraduation plate is described at pages 3 and 4. The device comprises a laser diode to produce a light beam which is split by a polarizing separator prism into two partial beams. The first partial beam traverses, by means of a first phase plate and a first mirror, a first angular graduation area of the graduation plate. As the first partial beam passes through the first angular graduation area, diffraction beams are produced. The positive diffraction beam of the first order is reflected, by means of a first reflector, back through the first angular graduation area. The beam is then deflected by means of the first mirror, through the first phase plate, and onto the polarizing separator prism. The polarizing separator prism directs the beam through a third phase plate, a beam separator and a first polarization plate, onto a first photoreceiver.
The second partial beam traverses, by means of a second phase plate and a second mirror, a second angular graduation area of the graduation plate. As the second partial beam traverses through the second angular graduation area, diffraction beams are produced. The negative diffraction beam of the first order is, by means of a second reflector, reflected through the second angular graduation area. The beam is then deflected, by means of the second mirror, through the second phase plate, and onto the polarizing separator prism. The polarizing separator prism directs the beam through the third phase plate, the beam separator, and a second polarization plate onto a second photoreceiver. A disadvantage of this device is that it is relatively complex and is expensive to assemble and adjust. An additional disadvantage is that only half of the light intensity of the laser diode reaches both of the photoreceivers.